The plane landed safely at Sea-Tac Airport just after 4 p.m.
A United Airlines spokesman said Flight 139 with 210 passengers on board was diverted after an indication of a problem with an oil filter and they are working to accommodate stranded passengers.
Flight tracking site FlightAware.com says the plane took off from Denver at 4:37 p.m. MDT and had made it to about the northern tip of Vancouver Island before turning around and heading for Seattle. A photo taken from inside the plane by Chris Seewald showed what he says was the airline dumping fuel over the Pacific Ocean in preparation for the landing into Seattle:
The incident comes on the same day that Boeing announced it was starting work on a stretched-out version of its 787 Dreamliner jet, in the hope of reigniting interest in the aircraft after battery-related problems.
Boeing announced the formal launch of its 787-10 program at the Paris Air Show on Tuesday and says it already has commitments from several customers, including United Airlines.